Selective amplifier or oscillator



Oct. 16, 1945. B. M. HADFIELD I 2,386,892-

SELECTIVE AMPLIFIER OR OSCILLATOR Filed June 5, 1942 2 Sheets-Sheet 1r-o 1, T iowrPur- KP -o FIGJ.

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SELECTIVE NEGATIVE FEEDBAcK LEAD BE BY v Oct. 16, 1945. B. M. HADFIELD2,386,892

SELECTIVE AMPLIFIER OR OSCILLATOR Filed June 5, 1942 2 Sheets-Sheet 2OUTPUT -P INVENTOR BERTRAM MORTON HADFIELD TAITOBNEY Patented Oct. 16,1945 SELECTIV E KMPLIFIER R OSCILLATOR Bertram Morton Hadfield,HarrowWeald, Engv land, assignor to Automatic Electric LaboratoriesInc., a corporation of Delaware 1 a Application June 5, 1942, Serial No.4431835 In Great Britain June 23, 1941 5 Claims. (Cl. 250-36) Thisinvention has reference to circuit arrangements for generating orselectively amp1ifying electrical oscillations of a given frequency. Oneof the objects of the invention is to provide a more simple means ofcontrol of the given frequency, by adjustment of the, amplitudes ofvoltages of. currents in the circuit. A further object is the useof asingle means of control,

such as a potentiometer, with which a frequency I range of some ten to.one may be obtained and having a close approximation to a logarithmicaccording to the method of construction of the whole circuit. Thesevoltages or currentsare then individually applied to gain-adjustingdevices and the gains adjusted so that at the desired frequency theoutputs are equal in magnitude. The outputs are then either added orsubtracted according as to whether they are out of I phase or in phaserespectively, in a common cirfrequency scale for linear-movements of the7 control. v 1

The type of generator or selective amplifier of electrical oscillationswith which this invention is concerned, is that in which a thermionicamplifierof .normal design has an incorporated feedback path which.Elves substantially no out. put at the desired frequency. {At otherfrequencies the output is-such as to provide a'degenerative effect, sothat its-the original gain of the amplifier be large then a sharplyselective frequency response curve is obtained. This form of selectiveamplifier iswell-known and it is also known that if a measureofregenerative feedback be also introduced such that the latter doesnotmaterially affect the selective characteristic of the amplifier, thenelectrical oscillations at 1 the desired frequency will be obtained.

In former examples of this type of apparatus, the feedback path whichprovides zero output at the desired frequency has consisted of circuit,

elements containing resistances and reactances and control of thedesired frequency has been obtained by simultaneously altering "groupsof the resistance or reactance elements. It will be seen that thismethod involves the use of elements which" are accurately matched andremain so over the degree of control required. In the presentinvention,although use of resistance and reactive elements is made in the feedbackpath. these elements are not used for control'purposes and they need nottherefore be accurately matched, provided'an initial adjustment is made.

Theqmode, of operation .of the feedbackpath the provision of twovoltages or feurrents, the

. magnitudes of whichvary in opposite sense with change of frequencyandwhich voltages and curin the-"present invention-consists essentiallyin quency. It will be seen that by suitable adjustment of the gaindevices the output of the feedback path can be made zero at anyfrequency,

"thus efiecting control of the latter. Further according to theinvention it is possible for two gain adjusting devices to be made intothe form of a single potentiometer, so that control isonly dependent onone adjustment. Also it will be apparent that the gain adjusting devicesmay be equall effective if they control theinputs to the circuits givingthe two voltages or currents. In order that a better appreciationof theinvention may be obtained, the following specific embodiments will nowbe described with reference to the accompanying drawings, in whichFigure 1 shows a simple form of feedback path employing an inductanceand .a capacity and embodying the present invention.

Figure 5 shows the complete circuit indiagrammatic form of a selectiveamplifier according to the present invention and Figures 6 and 7 showdetails oiyet other forms Y of feedback paths. Any one of. the circuitssuitable amplifier or oscillator with'the output terminals of saidfigures connected to the input terminals of the amplifier oroscillatorin the manner shown in Fig. 5.

able the objects'of the invention to'be under stood. Practicaldifficulties in using this form 7 i are described'later.

Fig. 1 shows a pure'inductance L and a pure capacity 0 connected inseries and supplied with a current I which is considered to be constantshownin Figs. 1, 4, 6 and 7 may be connected across the output of airrespective of the impedance variations of the circuit. Theinstantaneous polarity of the voltages is shown. Potentiometers areconnected across the inductance and capacity and the output is takenfrom the respective arms. The fractional settings are q and 11respectively. If w be the frequency of the current I and we theresonance frequency (his) both in radians per second it is obvious thatthe output is zero when qwL MC and from this we have .1: i we g -r thecurve has a point of inflection at the value so that the deviations fromany straight line drawn through this value for and a linear scale whichis obviously 0.5 are equal for equal ratios on either side thereof. If astraight line be drawn through 'settingvalues of 0.9 and 0.1, then thedeviations of the curve are negligible over a 10:1 frequency range, fromthe point of view of approximation to equal percentage reading accuracyover the range.

It has been assumed that the inductance L and capacity C are both freeof resistance components. Although close approximations to this state ofaifairs may generally be obtained in practice this embodiment is notpreferred owing to practical difliculties in maintaining such a state.

The above embodiment ensures that the working voltages will be 180 outof phase at all frequencies, if the two potentiometers, arrangedrespectively across the inductance and capacity, do not affect thevoltages as regards magnitude or phase. The relationship between thegiven frequency and the ratios on the potentiometers can be made tofollow any desired law; for instance. if the product p and q be keptconstant then there is a linear relationship with p and inversely linearwith q.

The relationship between p and q which is preferred, especially when amodification in the potentiometer circuits to be described is made, andwhich consists of making q equal to (1-p), confers the advantages of thelogarithmic form of scale as regards equal percentage reading accuracy,and at the same time the actual calibration of the potentiometers can bemade according to a known simple law.

As stated before the arms of the potentiometers could be coupledtogether for this type of relationship, and this leads to the preferredembodiment whereby a single potentiometer or gain control device isused. If a phase reversal is made in one of the voltages, by for examplea secondary winding Mi (Fig. 6) mutually coupled to the inductance thena single potentiometer P can be connected across the remote ends of theinductance and capacity voltages, and a position can always be found forthe arm where the voltage between the arm and the join of the voltagesis zero. The relationship between the two portions of the potentiometeris of the same form as before and therefore gives a substantiallylogarithmic variation of frequency for linear movements of the arm.

If the secondary winding has unity ratio then cutput=Vz.p(Vr.+Vc)=Vz.(lp) -pVc. This equals zero when L W170- and l LG- As pointed outheretofore, the methods of control may equally well be carried out onthe inputs to the circuits. For instance, the inductance and capacitymay each be fed with current of adjustable magnitude from separatetransformers T1 and T: (Fig. 7) whose primary windings are supplied witha common current and whose secondaries are tapped in accordance with anydesired relationship between frequency and tapping points so that thecurrents in the inductance and capacity may be adjusted to give equalvoltage outputs at the desired frequency. The tapping points oftransformers T1 and T2 are selected by switches SWI and SW2 such thatcurrents 1 and PI flow in L and C respectively where I may be theconstant primary current.

VL=1ULQI i 10C Therefore output is zero when P wLqI i. e. when Asregards the method of control by using one or more potentiometers on theoutput voltages of the feedback path the connection of suchpotentiometers without altering the magnitude and phase angles of thevoltages, may be effected in any well-known manner. In practice it isfound quite possible to use potentiometers of high impedance (say times)compared to the impedances of the circuit elements, but it is generallyadvisable to apply the output voltage to a thermionic valve arranged asa cathode follower so that the impedance of the circuit applied to theinput of the amplifier may be low; this being voltages with respect totheir common connection;

each type has two alternative methods of connection of the potentiometerwherein the latter may be used as a common internal gain control of thetwo stages or as an output gain control.- Only two versions will bediscussed.

Fig. 2 shows a. circuit for use where the voltages El and E2 are ofopposite sign with respect to the common connection (i. e. in series),and the potentiometer P4 is used as an output gain control. Valves VIand V2 are connected as cathode followers, having large cathodeimpedances Zi and Z2 respectively. Thus across the latter there will beavailable voltages corresponding to El and E2 but with the voltagesources now having internal resistances equal to the reciprocal .to theslopes of the valves. The potentiometer P4 is connected to the cathodesand the output is taken from the common connection and the arm; thecircuit C5, R5 constituting a conventional D. C. voltage eliminator.employing cathode follower valves, the internal resistancesbefore-mentioned will constitute a portion of the potentiometer, andmust be taken into account in the. choice of the resistance of thelatter and the setting determining the frequency for zero output.Variation of the internal resistance can be mitigated by making the endsof the potentiometer connected to the cathode into small semi-variableresistances, whose value swamps the internal resistances and can beindividually adjusted to make up a definite proportion of thepotentiometer in conjunction with the internal resistances. This methodis generally feasible because if the maximum frequency coverageislimited to :1, then there will be 0.09 of the true potentiometerateach end which is never used. This method is illustrated in Fig. 5where the true potentiometer consists of the-two internal resistances ofthe valves, RH! and R20, and P8. The latter can be designed to cover the10:1 range (1. e. true settings of 0.091 to 0.91) whilst R19 and R20 areadjusted to constitute "the remainder of the true potentiometer inconjunction with the internal resistances.

Fig. 3 shows a; circuit for use where the voltages E3 and E4 are ofsimilar sign with respect to the common connection (1. e. inopposition), and the potentiometer P5 is used as an internal gaincontrol. Valves V3 and V4 are connected as cathode followers, using theportions of P5 as cathode resistances. The gains of the valves will beinversely proportional to the resistances of their respective portionsof the potentiometer, except for the terminal and finite gainscorresponding to the slopes of the valves. Once again the latter can beconsidered as constituting an effective portion of the truepotentiometer and can be dealt with as before. The output is taken froma common anode resistance R6 by means of the centre tapped transformerTI and will be zero when the ratio of the gains is adjusted to be equalto the inverse of the input voltage ratio.

In Figure 3 this method of gain control is admissible in practice,because when the applied grid voltages become large then the gain mustbe reduced and hence the design or the circuit from the point of view ofgrid circuit overloading is facilitated. This latter method of gaincontrol is also possible when the grid voltages are or similar sign, theoutput then being taken from the anode circuits by means of a,transformer the primary of which is connected to the two anodes 3 and a.centre tap being taken to the positive battery.

It will be obvious to those skilled in the art that many othermodifications in the methods,

whereby a potentiometer or potentiometers may be used without affectingthe voltages applied thereto, are possible and it should be noted that.

In this and other circuits.

any of these methods may be applied to other forms of the feedbackcircuits other than the inductance and capacity type described. Anotherand preferred form of feedback circuit will now be described, with whichthe output at the desired frequency may be reduced to a zero in a mannersuperior to the inductance and capacity circuit because of theappreciable resistance components in practical inductances.

As stated before the object of the feedback path is to obtain twovoltages or currents of opposite sign independent of frequency and whosemagnitudes vary in opposite senses with frequency. Using a source ofhigh impedance and therefore of constant current it does not appear atpresent possible to obtain increasing magnitude with increasingfrequency unless inductances are used. Moreover since it is desirable tolet the output stage of the amplifier constitute the source, in orderthat harmonic production within the amplifier shall be reduced to aminimum (by being degeneratively fed back to the input via thefeedbackpath) then the output stage should preferably be of the lowimpedance constant voltage type, in order that an external load circuit'may be used without appreciably altering the voltage output.

Hence the form of feedback circuit which will be described withreference to Fig. 4 will presuppose a source of constant voltage, whoseimpedance is low compared with that of the circuit at any workingfrequency. In this form of feedback circuit illustrated in Fig. 4 thesource of alternating voltage is E and two seprate paths are used,comprising respectively resistances and capacities RP, Cl, R3, C3 andR2, C2 B4, C4 and fed from the common constant voltage source, such thatthe terminal voltage of one path rises with increase of frequency andthe terminal voltage of the other path falls with increase of frequency.The remaining requirement is that the terminal voltages shall be alwaysin or out of phase at any frequency. For instance, one path may consistof elements comprising series resistance and shunt capacity giving thefall-.

ing frequency characteristic, whilst the other comprises elements ofseries capacity and shunt resistance giving the rising frequencycharacteristic. In the design .of these paths two alterna-'capacity.feedback path before cited, in which the geometric meanfrequency of the working range correspond to the usual resonancefrequency.

It can readily be shown that if the time constants of each elementcomprising resistance and capacity in each path, and in the secondalternative that the resistances and capacities comprising a path arerespectively equal, then the terminal voltages of the two paths areeither in or out of phase at any frequency according to the polarity ofthe voltage applied to each path. Hence if each path has a commonconnection then the various potentiometer arrangements heretoforedescribed may be applied to the terminal voltages and the control of thefrequency at which zero output is obtained follows in a similar manner.

Furthermore it can be shown. that provided one component of each pathcan be adjusted, then the remaining components need only be ofcommercial accuracy designed to give time constants of the requiredorder. Actually this will only ensure that the real parts of thevoltages meet the requirements, by making the products of the timeconstants in each path equal to the reciprocal of the square of thedesired frequency in radians. In order to cater for the imaginary partsas well, it is necessary to be able to alter the actual time constantsin one part, whilst keeping their product constant. This can bearranged, to a 'sufflcient order of accuracy by converting the junctionof RI and R3 in Fig. 4 into a, potentiometer forming a small part of thetotal resistances. The remainder of RI and a suitable portion of R2 canthen be variable and form the aforementioned variable component in eachpath. It should be noted that a balance test conducted with thepotentiometer setting at its midpoint will be concerned with theimaginary parts and that a test at a setting at one end of thepotentiometer will be mainly concerned with the real parts. In thismanner it is easy to check the correct balance conditions. When the twopaths have been assembled complete with any one of the potentiometerarrangements, it is found that adjustment of the semi-variable componentin each path gives zero output at the desired frequericies according tothe setting of the potentiometer arrangement and as heretoforedescribed.

The location of the potentiometer arrangement may be at the outputvoltages of the two paths, at any convenient intermediate point in thepaths, or at the voltages applied to, the paths, since the only effectof the potentiometer is to provide a multiplying factor on the voltagespres-' ent in each path. For instance, the common point of the two pathsmay be connected to the arm of a potentiometer whose ends are connectedto the constant voltage source, thus giving control of the voltagesapplied to each path. This method is not preferred in practice, as thesource impedance for each path is varied according to the setting of thepotentiometer unless the potentiometer is of very low impedance, andconsequently the output isnot quite zero at the desired frequencies andpotentiometer settings other than that at which the paths have beenadjusted.

Fig. 5 gives a detailed circuit including a suggested form -of amplifiershown within dotted lines, which has been found to be satisfactory inpractice. In general this circuit may take the form of any amplifier ofconventional type, the

circuit C1 and RIO.

requirements bein an output impedance which is ne ligible compared withthat of the feedback paths, the phase of the output to be opposite tothe input in order that the feedback to the input shall be negative atall other frequencies, no self-oscillating frequency with feedback, areasonably constant amplitude response and negligible phase change overthe desired operational frequency band, and as large a gain as ispracticable. The latter is desirable as the selectivity of the wholecircuit is directly dependent upon the gain. In addition and when usedas an oscillator, there must be a positive feedback path.

The selective feedback circuit is comprised by CH, RH, CI3, RIB on oneside and RM, CHI, RIB, CI2 on the other side, being fed from the outputof the amplifier with alternating voltage (in the present case a D. C.eliminating circuit 08 and R2! is necessary, owing to the direct pathvia the lower side). The potentiometer circuit is of the form shown inFig. 2 where L2 and L3 ,replace Zl and Z2; RH and RIB constitute selfbias resistances for valves V8 and V9 and may be made up by the D. C.resistances of L2 and L3; As mentioned before RI! and R20 are smallvariable resistances designed to constitute the unused ends of the truepotentiometer in conjunction with the internal resistances of thevalves. The selective negative feedback lead is taken from the arm ofthe potentiometer P8 to the input of the amplifier via a D. C.eliminating circuit C6, R8.

The amplifier that is shown employs a valve of the high slope pentodetype for V5, but any convenient type may be used. The amplificationprovided by valve V5 is a maximum by using a high anode resistance R1and some cathode ne ative feedback from the resistance of thepotentiometer P1. The screen voltage is adjusted by P6 so that with thegrid bias provided, overloading takes place simultaneously on positiveand negative anode voltage excursions, being due in the positive senseto grid current in V6 and in the negative sense to anode overloading ofV5.

' V6 is arranged as a cathode follower, the cathode resistance R9 beingdesigned so that grid current shall take place at the appropriate anodevoltage of V5. The alternating voltage on R9 is fed to the grid of V1via C8 and RH. Vl constitutes the output stage and is also of cathodefollower type to give a minimum of internal impedance, the cathodeimpedance being LI and whatever load resistance is placed across theoutput terminals. The latter is limited by the necessity for ensuringthat grid current in V1 occurs at a higher voltage than in V6; R12 isthe self bias resistance for V1 and may be constituted by the D. C.resistance of LI. The positive feedback is taken from R9 to P1 via theD. C. eliminating This.path is preferably aperiodic in nature over theworking frequency range, if the frequency characteristics of theamplifier are also aperiodic; if not, then this path must be modified sothat the degree of positive feedback remains sumcient just to giveoscillation over the whole frequency range, when used as an oscillator.The positive feedback ratio may be adjusted by alteration of thepotentiometer P1. The input terminals A. B. which are shorted when inuse as an oscillator, enable the circuit to be used as a selectiveamplifier; the positive feedback being reduced to zero, of course by P1.

The valve V'l constitutes the low impedance source one terminal of whichis connected to a common busbar or return lead of the two selectivefrequency paths as heretofore described in connection with Figure 4,whilst the other terminal is connected to the remaining two input leadsof the two paths. The output voltages of the two paths are then ofopposite polarity with respect to the common busbar, and the latter mayform the negative busbar of the battery supplying the amplfier. Hence,provided the direct current conditions of the amplifier are met in anywell-known manner, a potentiometer may be connected across the outputleads of the two paths and the alternating output between the arm andthe busbar applied to the input of the amplifier, so that at frequenciesother than the desired frequency the feedback is degenerative. The formof the potentiometer may be of any of the appropriate types describedand the output therefrom may, if desired, be taken via an impedancetransforming device such as a valve arranged as a cathode follower. Inthe embodiment shown in Fig. 5 the coupling impedances of the amplifier,whether stray or intentional, are reduced to a minimum, and anyfrequency characteristic of the amplifier is mainly due to the straycapacity across the anode load resistance. If the grid bias of the firstvalve is derived from a cathode resistance, then it is possible toincrease the frequency at which this stray capacity is effective byplacing a shunt capacity across the cathode resistance; at the expense,of course, of the maximum gain. A value ofcathode capacity givingapproximately equal cathode and anode time constants is suitable. AlSoby designing the amplifier to overload simultaneously on equal positiveand negative output alternations only odd harmonies are appreciablygenerated, and the degenerative action of the feedback path becomes moreeffective in producing a sinusoidal output. Furthermore both overloadpoints are closely re lated with the battery voltage so that stabilityof output voltage will be ensured by the normal precautions applying tothe provision of a steady battery voltage. As is well-known for thistype of generator or selective amplifier, the output voltage and currentis only limited by the battery voltage and the type of output valve.

From the above description of the invention it will be seen that theaccuracy of control of the des red frequency is dependent on theaccuracy with which a potentiometer may be controlled and calibrated.

A ten to one range of frequency has been suggested as suitable forcoverage by the potentiometer, owing to the relatively small deviationfrom a logarithmic frequency scale. It will be apparent, however, thatby using further groups of feedback circuits designed for othergeometric mean frequencies the effective range of the invention can beexpanded at will, and provided these further ranges are made convenientmultiples then a common scale can be used. There is a further reason whyit is inadvisable to cover much more than a ten to one range with thepotentiometer, which should be borne in mind when deciding on theminimum gain of the amplifier. This is that the gain of the feedbackpath to frequencies other than the desired frequency varies with thepotentiometer setting. For instance, with a fractional setting of 0.5the maximum voltage output at very high or very low frequencies isobviously 0.5 of the input voltage. A loss of 6 db. therefore occurs inthe maximum possible degenerative effect of the amplifier and feedbackpath. Similarly with a setting of 0.1 or 0.9 a loss of 20 db. occurs atfrequencies respectively below or above the desired frequency (assumingthat frequency is proportional to these setting figures, as heretoforeshown). Provided that the amplifier has sufficient gain (say 50 db.)this efiect is not of reat importance as regards harmonic suppression,since the minimum degenerative effect can be kept to some 30 db.

The invention as described is not restricted in its applications to anyparticular band of frequencies, except by consideration normallyapplicable to the design of high gain amplifiers having feedback paths,that is, stray capacities and inductances and impurity of thecomponents. By using well-known technique therefore, the invention maybe applied to any desired range of frequencies and may be adapted foruse in wireless receiving or transmitting sets, or in carrier or audiofrequency telephony or telegraphy.

No difficulty has been found in the application of the invention apartfrom that common to all such similar devices, that is, the design of anamplifier to give a reasonable gain without having instability at highfrequencies when used with negative feedback. The circuit described isfree from this defect and is capable of a gain of 50 db. The coverage ofany number of frequency bands can be effected merely by changing thefixed condensers in the feedback path. No instability of voltage outputwas obtained even when using a battery supply operating switchgear.Frequency stability and waveform were as good as with other types ofoscillators in general use. The cost of this oscillator should beconsiderably less than most types as the variable elements are reducedto one which is of easily manufactured type, whilst the bulk iscorrespondingly smaller.

I claim:

1. An amplifier or oscillator including an input circuit and an outputcircuit, a circuit path associated with the output circuit for derivinga voltage therefrom which varies in one sense with changes in frequency,another circuit path associated with the output circuit for derivinganother voltage therefrom which varies in the opposite sense withcorresponding changes in frequency but remains in phase opposition tothe first voltage, adjustable means for combining the derived voltagesin variable proportions and applying the resultant voltage to the saidinput circuit to provide a degenerative feedback which becomes zero atany desired frequency determined by the adjustment of said means wherebythe oscillator frequency or the frequency to which the amplifier isselective may be controlled by said means.

2. An oscillator having a regenerative feedback, means for deriving avoltage from the oscillator which increases with an increase of thefrequency therein, means for deriving a second voltage from theoscillator which decreases with an increase of the frequency therein, acircuit for combining the derived voltages and applying the resultantvoltage to the input as a degenerative feedback, the circuit forcombining the derived voltages comprising a potentiometer so connectedthat the feedback may be reduced to zero at any desired frequency byadjustment of said potentiometer.

3. An amplifier having an input and an output, a circuit for obtaining avoltage from the output which increases with an increase in thefrequency of the current flowing in the amplifier, and for obtaininganother voltage which decreases with in: in the anipliner. anothercircuit for combining the voltages obtained and applying the resultantvoltage becoming zero at any desired sultantsvoltage to the input as adegenerative feedback, said other circuit for combining the derivedvoltages comprising a potentiometer so connected that the feedback maybe reduced to zero at any desired frequency by adjustment of 7 saidpotentiometer.

4. A network comprising inductance and capacitance, or resistance andcapacitance, elements wh y be connected to a source of varyingfrequency, deriving two voltages therefrom which vary in magnitude inopposite senses with changes in frequency while maintaining the samephase relationship, means for applying the derived voltages in series toa potentiometer and obtaining a resultant voltage from an adjustable tapon said potentiometer and a common connection for the two derivedvoltages, said rego frequency depending on the adjustment of saidpotentiometer. v

5. An amplifier or oscillator having an input and an output, a circuitarrangement of condensers'and resistors associated with the output forderiving two voltages therefrom which vary in magnitude in oppositesenses with changes in frequency while maintaining the same phaserelationship} means in bridge of said condensers and resistorsfor addingthe voltages and applying the resultant voltage to the input to provideeither a zeroor degenerative efiect according to the frequency, saidmeans comprising a potentiometer the adjustment of which determines thefrequency at which the zero efiect occurs and thus determines theoscillator frequency or the frequency to which the amplifier isselective.

BERTRAM MORTON HADF'IELD.

